NO. 2 CAMBRIAN FOSSILS FROM MOHAVE DESERT RESSER 5 



Pacdumias, as it is proposed to restrict it, possesses several definite 

 generic characters. The glabella fails to reach the rim and is con- 

 nected with it by a ridge that crosses the intervening space. The 

 marginal sutures and rim are quite like the same features in Mesonads. 

 All the species strictly referable to Pacdumms thus far studied 

 possess intergenal spines, a feature not present in the types of 

 Mesonads or OlcneUus. 



Except that intergenal spines have not been found in Nevadia, the 

 cephala of that genus and Pacdumias have a number of characters in 

 common. In both, the glabella does not reach the rim and is connected 

 medially with it by a ridge. In both genera the glabella tends to taper 

 forward, a feature that has caused the placing of a number of species 

 into Callavia that do not belong there (see p. 6). 



SYNOPTICAL CHARACTERIZATION OF MESONACIS, OLENELLUS, 

 PAEDUMIAS, NEVADIA, AND CALLAVIA 



The characteristics of the various genera studied in connection with 

 this paper may be briefly summarized as follows : 



Mesonads. — Glabella touches the anterior rim and does not taper 

 forward, but usually has more or less of an hour-glass shape. Rim 

 narrow and striated. Marginal and epistomal plates separated by 

 intramarginal and marginal sutures. Third thoracic segment large, 

 and a strong spine occurs on the fifteenth. Rudimentary segments, 

 with well developed pleurae, posterior to the fifteenth. Hypostoma 

 without spines. 



Olenellus. — Glabella as in Mesonads. Rim perhaps a little wider 

 and eyes somewhat longer. Thorax same as in Mesonads, to fifteenth 

 segment. Rudimentary segments posterior to the fifteenth without 

 definite pleurae, i. e., without dorsal furrows. 



Paedumias. — (Restricted) Glabella usually tapers forward, never 

 extends forward to rim, with which it is connected by a median ridge. 

 Rim, marginal and epistomal plates, and thorax to the fifteenth seg- 

 ment like Mesonads and Olenellus. Intergenal spines present. Rudi- 

 mentary segments posterior to the fifteenth zvifJiout pleurae, as in 

 Olenellus. Hypostoma with spines on posterior margin and connected 

 with the rostral or epistomal plate by a stalk, which probably causes 

 the median ridge on the upper surface of the cephalon. 



Nevadia. — Cephalon most like Paedumias. Glabella fails to reach 

 the rim, with which it is connected by a median ridge. Rim and 

 sutures possibly the same also, but none of the specimens is well 

 enough preserved to be quite certain on these points. No intergenal 



